1. Field of the Invention
This invention relates to glass-coated amorphous or nanocrystalline alloy microwire for electronic article surveillance and other systems; and more particularly to articles composed of such microwires, which have multi-bit encoding/reading capabilities.
2. Description of the Prior Art
Electronic Article Surveillance (EAS) systems are used to electronically detect goods that have not been authorized when they are removed from a retailer. The systems comprise a marker attached to the goods and a sensor mechanism. The retailer can neutralize the marker when he wishes to authorize the removal of the goods, for example when the items have been legitimately purchased.
One type of EAS marker, termed harmonic or electromagnetic, is disclosed by U.S. Pat. Nos. 4,484,184 and 5,921,583. Such a marker comprises a plurality of strips or wire segments of ferromagnetic amorphous magnetic material that resonate electromagnetically and thereby generate harmonics in the presence of an incident magnetic field that has a preselected frequency and is applied within an interrogation zone. Generation of harmonics under these conditions provides marker signal identity. An unmagnetized permanent magnet in the vicinity of the resonating plurality of strips or wire segments can be magnetized or demagnetized to inactivate or re-activate said marker.
Another type of marker, described as being acoustomagnetic or magneto-mechanical, is disclosed by U.S. Pat. No. 4,510,490. In this system, a marker is adapted, when energized, to mechanically resonate at preselected frequencies that are provided by an incident magnetic field applied within an interrogation zone. The marker has a plurality of elongated ductile strips of magnetostrictive ferromagnetic material. Each of the strips is disposed adjacent to a ferromagnetic element which, upon bring magnetized, magnetically biases the strips and energizes them to resonate at the preselected frequencies. A substantial change in effective magnetic permeability of the marker at the preselected frequencies provides the marker with signal identity.
The prior art technologies described hereinabove provide a method and means for sensing the presence of an object to which a marker is affixed.
In related technologies, multi-bit rather than single-bit (on/off) markers have been described in the technical and patent literature. For example, a publication by Zhukov et al., J. Mater. Res. 15 No. 10 Oct. (2000), reports on the ability to produce a multi-bit marker when utilizing multiple amorphous glass-coated wire segments, each having a different dimension (length, alloy vs. gross diameter, etc.) or magnetic property (coercive field, etc). A multi-bit marker disclosed by U.S. Pat. No. 5,729,201 to Jahnes describes a similar marker containing multiple wires; but wherein all wires have the same chemistry and geometric dimensions. A permanent magnet bias field element in the vicinity of an array of amorphous metallic wires serves to differentiate the drive field, at which harmonic response is obtained, by way of proximity of each individual wire segment to the permanent magnet bias field element, thereby providing multi-bit capability. PCT patent publication WO 01/29755 A1 to Antonenco et al. describes a multi-wire marker that is capable of multi-bit performance. As with the Jahnes teaching, each of the microwire segments utilized in the construction of the marker have the same chemistry and geometric dimensions. Antonenco et al. disclose arranging the microwires in a manner similar to stripes in a conventional optical bar code. Information concerning the Antonenco et al. marker is read using a magnetic reading head.
Each of the encoded markers described in the technical and the patent literature requires the use of a plurality of magnetic elements (strips or wire segments). These multiple magnetic elements must be carefully arranged with respect to each other. They increase the size, weight and cost of the marker and, unless accurately positioned thereon, decrease its identifying characteristics.
There remains a need in the art for a glass-coated amorphous or nanocrystalline alloy microwire marker that is light weight, small, inexpensive to construct and highly reliable in operation.